An orthogonal frequency division multiplexing (OFDM) divides high rate data stream to a plurality of low rate data streams and transmits these two types of data streams by using a plurality of carriers. Here, each of the plurality of carriers is called a subcarrier. Since orthogonality exists between each of the plurality of carriers in OFDM, the receiving end can still detect and decode even if the frequency characteristics of the carriers are overlapped. The high rate data stream is passed through a serial-to-parallel (S/P) converter and converted into the low rate data stream. Thereafter, the subcarriers are multiplied the converted data streams, and each data stream is added and transmitted to the receiving end.
The data streams converted by the S/P converter can be allocated to a plurality of subcarriers after being applied an Inverse Discrete Fourier Transform (IDFT). Here, an Inverse Fast Fourier Transform (IFFT) can be used in place of the IDFT.
Since symbol duration of the subcarriers carrying low data rate stream increases, signal dispersion in time domain caused by multi-path delay spread decreases relative to the symbol duration. A guard interval that is longer than channel delay between the OFDM symbols in order to reduce inter-symbol interference (ISI). Furthermore, a part of the OFDM signal is placed in the guard interval and place the guard interval in front portion of the symbol to protect the symbol by the OFDM symbol becoming cyclically extended.
An OFDM Access (OFDMA) refers to a method for multiple access which provides available subcarriers to each user in a system using the OFDM for modulation. In other word, the OFDMA scheme allocates frequency resources (e.g., subcarriers) to data symbols associated with each user, and each of these frequency resources is independently allocated so as to prevent overlapping and interference. Simply put, the frequency resources are allocated mutually exclusively.
A Multiple Carrier Code Division Multiple Access (MC-CDMA) refers to another method of preventing interference between data symbols. More specifically, the MC-CDMA scheme spreads data symbols by multiplying different codes to the data symbols and allocating them across a frequency domain so that the data symbols are distinguishable by the receiving end.
However, the OFDMA and the MC-CDMA schemes have following issues to be resolved. With respect to the conventional OFDMA scheme, as discussed above, each frequency domain is allocated to the each user and not shared with other users. As such, if a serving cell allocates a first frequency domain to a first user, serving cells neighboring the first serving cell cannot use the first frequency domain of its respective cells due to interference from using the same frequency domain. To resolve this conflict, the neighboring cells cannot use the same frequency domain. The shortcoming of this type of resource avoidance is that frequency reuse factor decreases.
With respect to the conventional MC-CDMA scheme, as discussed above, the data symbols are spread across the entire frequency domain. As such, in a frequency selective channel environment, an equalizer must be used and thus, frequency diversity cannot be fully utilized and result in capability deterioration.